Retention of Structural Cores in the Synthesis of High-Nuclearity Polyoxoalkoxomolybdate Clusters Encapsulating [Na(H₂O)₃]⁺ and [MoO₃] Moieties

Hydrothermal reactions of molybdenum-oxide precursors with polyalcohols in the presence of base yielded two series of mixed-valence oxomolybdenum clusters, the hexadecanuclear species [XH₁₂(Mo^VIO₃)₄Mo^v ₁₂O₄₀]^m- (X = Na⁺, m = 7; X = 2H⁺, m = 6) and the superclusters [XH_nMo^VI₆Mo^V₃₆O ₁₀₉{(OCH₂)₃CR}₇]^m- (X = Na-(H₂O)₃⁺, m = 9, n = 13; X = Na(H₂O)₃⁺, m = 7, n = 15; X = MoO₃, m = 9, n = 14; X = MoO₃, m = 10, n = 13). In a representative synthesis for the hexadecanuclear class of materials, the hydrothermal reaction of a mixture of Na₂MoO₄·2H₂O, MoO₃, Mo metal, and NH₄Cl produced (NH₄)₇[NaMo₁₆(OH)₁₂O ₄₀]·4H₂O (1·4H₂O) as red-orange crystals. The compound (Me₃NH)₄K₂[H₂Mo ₁₆(OH)₁₂O₄₀]·8H₂O (2·8H₂O) was prepared in a similar fashion. The structure of the anion of 1 consists of an ε-Keggin core {H₁₂Mo₁₂O₄₀}, capped on four hexagonal faces by {MoO₃} units and encapsulating a Na⁺ cation. The structure of the oxomolybdenum framework of 2 is essentially identical to that of 1; however, the central cavity is now occupied by 2H⁺. The synthesis of the "superclusters" exploits similar hydrothermal conditions, resulting in the isolation of (Me₃NH)₂(Et₄N)Na₄[Na(H ₂O)₃H₁₅-Mo₄₂O ₁₀₉{(OCH₂)₃CCH₂OH} ₇]· 15H₂O (3· 15H₂O), (Me₃NH)₂(H₃O)Na₆[Na(H ₂O)₃H₁₃Mo₄₂O₁₀₉{(OCH ₂)₃CCH₂- OH}₇]·7H₂O (4·7H₂O), Na₉[(MoO₃)H₁₄Mo₄₂O ₁₀₉{(OCH₂)₃CCH₂OH} ₇]·0.5C(CH₂OH)₄· 25H₂O (5·0.5C(CH₂-OH)₄-25H₂O), and (Me₄N)(Et₂NH₂)(H₃O) ₂Na₆ [(MoO₃)H₁₃Mo₄₂O₁₀₉{(OCH ₂)₃CCH₃}₇]·10H₂O (6·10H₂O). The structures of these superclusters resemble a bowl consisting of an oxomolybdenum framework, constructed from eighteen pairs Mo(V) - Mo(V) dimers and linked through edge-sharing of {MoO₆} octahedra, with six cis-dioxomolybdate(VI) sites protruding outward to provide the surface of a channel to the molecular cavity of the complex anion. The cavity is populated by a Na(H₂O)₃⁺ group in 3 and 4 and by a {MoO₃} unit in 5 and 6. Crystal data are as follows. 1·4H₂O: cubic F4̄3m (No. 216), a = 26.978(3) Å, Z = 8. 2·8H₂O: orthorhombic Cmcm (No. 63), a = 18.493(4) Å, b = 19.685(4) Å, c = 19.518(4) Å, Z= 4. 3·15H₂O: triclinic P1̄ (No. 2), a = 22.159(4) Å, b = 27.049(5) Å, c = 17.726(3) Å, a = 98.34(1)°, β= 112.56(2)°, γ = 82.81(1)°, Z = 2. 4·7H₂O: monoclinic C2/c (No. 15), a =_54.901(11) Å, b = 17.616(4) Å, c = 36.571(7) Å, β= 94.04(2)°, Z = 8. 5·0.5C(CH₂OH)₄·25H₂O: triclinic P1̄ (No. 2), a = 17.302(3) Å, b = 20.865(4) Å, c = 26.734(5) Å, α = 103.90(3)°, β= 99.26(3)°, γ = 108.95(3)°, Z = 2. 6·10H₂O: orthorhombic Pbcm (No. 57), a = 17.916(4) Å, b = 44.614(9) Å, c = 23.472(5) Å, Z = 4.